The present invention relates to a high-Cr ferritic, heat-resistant steel which contains Cu and which has improved resistance to copper checking in addition to good high-temperature strength and toughness. More particularly, it relates to such a ferritic steel which is substantially free from copper checking during hot working and which is suitable for use in various high-temperature parts required to withstand both high temperatures and high pressures such as steel tubing and piping, steel sheet for pressure vessels, and materials for turbines in a wide variety of industrial applications such as boilers, chemical plants, and nuclear facilities.
Heat-resistant steels for use in heat- and pressure-resistant high-temperature parts for boilers, chemical plants, nuclear facilities, or the like must have excellent high-temperature strength, resistance to hot corrosion and oxidation, and toughness, yet they must exhibit good workability and weldability, and it is also desirable that they be economical.
Conventional steels for use in such applications include (1) austenitic stainless steels such as ASTM TP 321H and TP 347H, (2) low-alloy steels such as 21/4Cr-lMo steel, and (3) high-Cr ferritic steels containing 9-12% Cr by weight. High-Cr ferritic steels are advantageous in that they are superior to low-alloy steels in respect to strength and resistance to hot corrosion and oxidation at temperatures in the range of 500.degree.-650.degree. C. while they are free from stress corrosion cracking, which is unavoidable in austenitic stainless steels. Furthermore, compared to austenitic stainless steels, high-Cr ferritic steels are less expensive and have a higher thermal conductivity with a lower coefficient of thermal expansion, so they are improved in resistance to thermal fatigue and are less susceptible to peeling.
Typical high-Cr ferritic steels which have conventionally been used include 9Cr-lMo steel (ASTM T9), modified 9Cr-lMo steel (ASTM SA213 T91), and 12Cr-lMo steel (DIN X20CrMoWV 121). For the purpose of improvement in high-temperature strength, it has been proposed to modify these steels by adding one or more elements selected from Mo, W, V, Nb and N. See, for example, Japanese Patent Publication No. 57-36341(1982), No. 62-8502(1987), and No. 62-12304(1987), and Japanese Patent Application Laid-Open No. 59-211553(1984), No. 61-110753(1986), No. 62-297435(1987), and No. 2-310340(1990).
In U.S. Pat. No. 5,069,870 and Japanese Patent Application Laid-Open No. 3-97832(1991), some of the present inventors proposed a high-Cr ferritic, heat-resistant steel having a Cu-containing novel composition on the basis of a finding that the addition of Cu is effective for improving the resistance to high-temperature oxidation at temperatures of suppressing the formation of .delta.-ferrite, which is caused by the presence of Cr in an increased amount. Therefore, the amount of Ni, which has conventionally been added for the same purpose, can be decreased, and as a result, the material costs can be decreased without a decrease in the thermal conductivity of the steel.
In the Japanese journal Current Advances in Materials and Processes, Vol. 4, No. 3, p. 884 (1991), it is reported that the addition of Cu has an effect of suppressing the formation of .delta.-ferrite in weld zones of a high-Cr ferritic steel, thereby improving the toughness in those zones. Likewise, Japanese Patent Application Laid-Open No. 2-294452(1990) describes a Cu-containing, high-Cr ferritic, heat-resistant steel which has improved toughness in weld zones by the above-described action of Cu.
As discussed above, many modifications have been made to high-Cr ferritic, heat-resistant steels which contains at least 9% by weight of Cr. However, the steel compositions heretofore proposed for these steels are still unsatisfactory with respect to at least one of toughness, stability of the structure, workability, and weldability, as described below.
(1) The weldability and workability of a high-Cr ferritic steel can be improved by decreasing the C content thereof. However, decreasing the C content is accompanied by the formation of .delta.-ferrite in a large amount in the base metal and/or the weld zones of the steel, resulting in losses of toughness and high-temperature strength.
(2) The addition of a relatively large amount of Ni, which is known to be effective in suppressing the formation of .delta.-ferrite, not only decreases the thermal conductivity of the steel and raises the cost thereof, but also accelerates the coarsening of carbide precipitates during use at high temperatures, resulting in a decrease in high-temperature creep strength.
(3) When Cu is added in order to suppress the formation of .delta.-ferrite, the simultaneous addition of a slight amount of Mg is advantageous from the viewpoint of avoiding a deterioration in workability, which is caused by the addition of Cu, as disclosed in the afore-mentioned U.S. Pat. No. 5,069,870. However, since Mg is difficult to melt, it is difficult to prepare such an Mg-containing steel by melting.
(4) The workability of a Cu-containing steel can also be improved by allowing a small portion of .delta.-ferrite phases to remain in the steel, as disclosed in Japanese Patent Application Laid-Open No. 3-97832(1991), in place of the addition of a slight amount of Mg. Such a steel in which slight amounts of .delta.-ferrite remain, however, has a decreased toughness, particularly in weld zones.
(5) The so-called copper checking phenomenon generally occurs in steels which contain a relatively large amount of Cu. Copper checking is caused by intergranular precipitation of Cu phases at high temperatures and results in cracking during working. Copper checking of Cu-containing steels can be avoided by the addition of Ni in an amount of at least 50% by weight of the Cu content. This measure is satisfactory with low-alloy steels, but the addition of such a large amount of Ni to high-Cr steels does not solve the problem mentioned in (2) above.